Mean runoff ratio (f_R) is defined as a ratio dividing runoff height (R_F) by annual precipitation (P), namely f_R=R_F/P. Relation between annual precipitation and runoff ratio is expressed by three ways ; f_R=a₀ (constant), f_R=a₁P and f_R=a₂Pⁿ (n<1). The case of constant f_R includes an essential consistency that the runoff is generated even in very small amount of P. The latter two cases are not unreasonable, and the justification of them are discussed in this paper. Runoff height for the latter ones are obtained by R_F=a₁P² and R_F=a₂P⁽ⁿ⁺¹⁾. Runoff ratios for the Tama River, the Sumiyo River and data from Selles’ textbook (1965) are summarized. Relation between annual precipitation and runoff height deduced from Kayane and Takeuchi (1971), Otsuki (1986) and Nakano (1974) are also summarized. These data suggest that runoff height may have the form of R_F=a₂(P-Em)⁽ⁿ⁺¹⁾, where Em is the minimum evapotranspiration. This means that f_R has the form of (P-Em)ⁿ, and fitted curve for river basins in Japan is f_R=0.0048 (P-500)^O.67, for Budyko’s data is f_R=0.017(P-200)^0.5.